Bates was quite right; you are the man to apply to in a difficulty. I never heard anything more ingenious than your suggestion (The suggestion that conspicuous caterpillars or perfect insects (e.g. white butterflies), which are distasteful to birds, are protected by being easily recognised and avoided. See Mr. Wallace's 'Natural Selection,' 2nd edition, page 117.), and I hope you may be able to prove it true. That is a splendid fact about the white moths; it warms one's very blood to see a theory thus almost proved to be true. (Mr. Jenner Weir's observations published in the Transactions of the Entomolog. Soc. (1869 and 1870) give strong support to the theory in question.) With respect to the beauty of male butterflies, I must as yet think it is due to sexual selection. There is some evidence that dragon-flies are attracted by bright colours; but what leads me to the above belief is, so many male Orthoptera and Cicadas having musical instruments. This being the case, the analogy of birds makes me believe in sexual selection with respect to colour in insects. I wish I had strength and time to make some of the experiments suggested by you, but I thought butterflies would not pair in confinement. I am sure I have heard of some such difficulty. Many years ago I had a dragon-fly painted with gorgeous colours, but I never had an opportunity of fairly trying it. The reason of my being so much interested just at present about sexual selection is, that I have almost resolved to publish a little essay on the origin of Mankind, and I still strongly think (though I failed to convince you, and this, to me, is the heaviest blow possible) that sexual selection has been the main agent in forming the races of man. By the way, there is another subject which I shall introduce in my essay, namely, expression of countenance. Now, do you happen to know by any odd chance a very good-natured and acute observer in the Malay Archipelago, who you think would make a few easy observations for me on the expression of the Malays when excited by various emotions? For in this case I would send to such person a list of queries. I thank you for your most interesting letter, and remain, Yours very sincerely, CH. DARWIN. CHARLES DARWIN TO A.R. WALLACE. Down, March [1867]. My dear Wallace,